This invention relates to an apparatus and method for tentering sheet-like members and more particularly this invention relates to a tentering frame and method for tentering membranes used in electrolyzers.
Electrolyzers employing a membrane (hereinafter "membrane cells") may be of the filter press type, for example, as described in U.S. Pat. Nos. 4,108,742 and 4,111,779. Membrane materials commonly used for membrane cells include, for example, those marketed by E. 1. duPont de Nemours & Company under the trademark Nafion.RTM. and by Asahi Glass Company Ltd. under the trademark Flemion.RTM.. The membranes are available principally in sheet-like form. The membrane is used for separating the cell into electrode compartments containing electrolyte. For example, a membrane cell used for the production of a halogen and an alkali metal hydroxide may use an ion exchange membrane to separate an anode compartment containing anolyte and an anode member from a cathode compartment containing catholyte and a cathode member.
In a membrane cell used, for example, in the production of a halogen and an alkali metal hydroxide, it is important to keep the distance between electrodes to a minimum to reduce the voltage drop through the catholyte and anolyte, and thus reduce energy consumption of the cell. Furthermore, it is advantageous to keep a uniform spacing between an electrode and the membrane to obtain a uniform current distribution. Any contact between the membrane and an electrode may cause a great amount of current passage and membrane burning at the point of contact. In some applications, the spacing between an electrode and membrane may be no greater than 1 millimeter. Therefore, the membrane is kept as flat or planar as possible when installed between electrodes of an electrolytic cell.
Some membrane materials are known to absorb water and expand a certain percentage when wetted. Thus, it is common for these types of membranes to form wrinkles during operation of a cell when the membrane is wetted with electrolyte. A wrinkled membrane can come into contact with the cell electrodes and cause the problems described above. A wrinkled membrane can also reduce the circulation of the electrolyte and trap gases produced in the cell between the electrode and the membrane face. This may result in a nonuniform increase in resistivity of the electrolyte solution in the interelectrode space with a nonuniform current distribution across the membrane surface in the vertical direction. It is important, therefore, to keep the membrane as flat as possible and prevent wrinkles from forming on the membrane surface when installing in the cell.
Heretofore, the installation of membranes between electrode compartments required a crew of about six to eight people holding the membrane in place and pulling the membrane by hand to tension the membrane between electrode units until the electrode compartment units were squeezed together by, for example, a hydraulic ram. Invariably, this procedure led to formation of wrinkles on the membrane due to uneven forces pulling at the membrane by the crew. The wrinkles formed at a gasket bearing surface of the membrane caused leakage of electrolyte into the atmosphere or electrode compartments. Furthermore, installation of the membrane consumed a relatively long period of time and dropping the membrane, which meant starting the installation process over, was always a risk.
It is desired to minimize the problems discussed above by providing an apparatus and method for tentering a membrane used in a membrane cell and maintaining the membrane planar when installed in a membrane cell.